Secondary or rechargeable lithium-ion batteries may be used as electric storage systems for powering electric and hybrid electric vehicles. These batteries comprise a plurality of suitably interconnected electrochemical cells each of which undergoes a specific chemical reaction capable of generating electrical energy. When suitably arranged, these cells provide a predetermined electrical current at a specified electrical potential to an external load, such as an electric motor. Such a battery may be re-charged by supplying electrical energy to the battery to reverse the chemical reaction undergone at the electrodes and render the battery again capable of delivering electrical power. Of course, there is continuing interest in higher capacity cells which enable extended use before recharging is required.
In principle, this cycle of charge and discharge may be continued indefinitely, but in practice, each cycle is less than fully reversible and so the capacity of the battery will be reduced or ‘fade’ with continued use. After some period of use, or some number of charge discharge cycles, the extent of fade or capacity reduction will be sufficient to render the battery unsuitable for its intended application and require that it be replaced.
In each cell of a lithium battery, on discharge, lithium is transported as lithium ions from a negative electrode through a non-aqueous, lithium-containing, electrolyte solution to a lithium-accepting positive electrode as an electronic current is delivered from the battery to an external load, for example, in a vehicle, an electric traction motor. A suitable porous separator material, infiltrated with the electrolyte solution and permeable to the transport of lithium ions in the electrolyte, is employed to prevent short-circuiting physical contact between the electrodes.
Graphite has been commonly used as a negative electrode material in such batteries and is commonly employed as a thin electrode layer bonded to a copper current collector. During charging of the cells, lithium is inserted into the graphite (lithiation, forming LiC6, with a capacity of about 372 mAh/g) and extracted from the graphitic carbon during discharging (de-lithiation).
A suitable particulate material for receiving and storing inserted lithium during discharge of each cell is used as the positive electrode material. Examples of such positive electrode materials include lithium cobalt oxide (LiCoO2), a spinel lithium transition metal oxide such as spinel lithium manganese oxide (LiMn2O4), a lithium polyanion such as a nickel-manganese-cobalt oxide [Li(NixMnyCoz)O2, where x+y+z=1], lithium iron phosphate (LiFePO4), or lithium fluorophosphate (Li2FePO4F), or a mixture of any of these materials. Suitable positive electrode materials are often bonded as a thin layer to an aluminum current collector. The electrochemical potential of such lithium ion cells is typically in the range of about 2 to 4.5 volts.
The use of lithium-ion batteries to power electric motors in automotive vehicles has led to the need for higher gravimetric and/or volumetric capacity batteries. While graphitic carbon is a durable and useful lithium-intercalating, negative electrode material for lithium-ion cells, it has a relatively low capacity for such lithium insertion. Other potential negative electrode materials such as silicon (theoretical capacity, 3578 mAh/g for Li15Si4) and tin (theoretical capacity, 994 mAh/g for Li22Sn5) have much higher theoretical capacities than graphite for lithium insertion.
However, unlike graphite, silicon undergoes a volume change that can exceed 300 volume percent during the course of lithiation and reverses during de-lithiation. Tin exhibits similar behavior. Such dramatic volume changes may induce, in the lithiated silicon, appreciable stresses which may lead to fracture of the active silicon material and/or loss of electrical contact by the silicon and its current collector. This loss of contact is manifested by a rapid reduction in the electrical storage capacity of the battery; that is rapid fade.
Loss of battery capacity resulting from the fracture of the electrode materials in its cells may result from loss of electrical contact with conductive material and the creation of new surfaces which irreversibly consume the active lithium to form new solid electrolyte interfaces. And, of course, any lithium entrained within the fractured, separated electrode material is irretrievably lost.
Thus there remains a need for a more effective way of utilizing high energy capacity negative electrode materials such as silicon or tin to enable development of a high-capacity, fade resistant lithium ion battery.